1. Field of the Invention
The present invention relates to an optical information recording medium provided with phase grooves in land portions of information recording areas thereof.
2. Discussion of Background
FIG. 1(a) schematically shows a format structure of a conventional information recording medium, which is composed of a synchronizing area for repeated patterns of reference clocks and for a phase lock loop (PLL) at reproduction, an identification area for registering address numbers at reproduction, and track numbers, a flag area for recording handling information when reproducing recorded information, and a data area for recording the user's data.
The synchronizing area, the identification area, and part of the flag area (except a recording area) are generally called preformat areas, and are preformed in the form of concave portions and convex portions (so-called prepits) in a substrate of an optical information recording medium or disk.
Part of the flag area, which is a recording area, and the data area are for the user's recording areas.
FIG. 1(b) schematically shows the flag area in more detail. The flag area is composed of four flag marks, Flag A area, Flag B, Flag C, and Flag D. These marks are made in the same pattern as shown in FIG. 3(b). Flag A is the mark for indicating that recording has been carried out in the data area. Flag B is the mark for indicating that the data has been verified and judged that there is no good sector. Flag C is the mark for indicating that the sector judged no good has been replaced. Flag D is the mark for indicating that data has been deleted.
Each of these flags serves as a system information when reproducing the information recorded in the data area, so that if any of these four flags is erroneously detected, the recorded data cannot be reproduced correctly.
For instance, when Flag A is erroneously detected, it is judged that no recording has been made in the data area, so that double recording takes place if recording is performed, and it becomes impossible to reproduce a previously recorded information, thus causing a fatal error.
A conventional method for reproducing data, which includes the data in the synchronizing area, identification area, flag area, and data area, will now be explained with reference to FIGS. 2(a) and 2(b).
FIG. 2(a) is a schematic illustration of an optical system for reproducing information from an optical information recording disk. A laser beam emitted from a photodiode 1 is changed to parallel laser rays by a collimator lens 2 and is then caused to pass through a .lambda./2 letardation plate 3 for adjusting a plane of polarization, a polarizing beam splitter (PBS) 4, a .lambda./4 letardation plate 5 for shifting the plane of polarization by .lambda./4, a pickup 6 for focusing the laser beam on a recording surface of an optical information recording disk 7 and having the laser beam track the guide grooves formed on the recording surface of the optical information recording disk 7. The laser beam reflected by the recording surface is again caused to pass through the pickup 6 and the .lambda./4 letardation plate 5, is then reflected by the polarizing beam splitter 4, and is then caused to pass through a cylindrical lens 8 for detecting a focus error signal by the astigmatism method, and enters a four-divided photodetector 9. Each signal detected by the four-divided photodetector 9 is converted into a current-voltage signal, and is then amplified so as to have an appropriate voltage by a system as shown in FIG. 2(b). The four signals from the four-divided photodetector 9 are added, so that an Rf signal is produced. The thus produced Rf signal is for detecting the changes in the quantity of the light reflected in accordance with the information on the recording surface, including prepits formed on the substrate of the optical information recording disk 7, and recording pits formed in the recording layer of the optical information recording disk 7 by the application of the laser beam thereto. The Rf signal is then subjected to an AC coupling, and the changes in the direct current level are removed, which include the changes in the quantity of reflected light caused by (a) the changes in the thickness of the recording layer, (b) the focus shifting, which is caused by the warp of the substrate of the optical information recording disk 7, and/or (c) the tracking shift, which is caused by the eccentricity of the optical information recording disk 7. The Rf signal is then converted into a binary digital signal by a zero-level slicer. By decoding the binary digital signal, a zero (0) or one (1) information can be obtained.
A method for detecting flag marks in a land recording method will now be explained with reference to FIG. 3(a), FIG. 3(b), and FIG. 3(c). FIG. 3(a) shows the state of no flag marks having been recorded. FIG. 3(b) shows the state of flag marks having been recorded and therefore being normally detectable. FIG. 3(c) shows the state of flag marks having been recorded, but having been erroneously detected.
In this land recording method, prepits and recording pits are formed in a portion called "land" between adjacent guide grooves which are preformed in a substrate of an optical information recording disk. The prepits and recording pits are situated in the central position between the adjacent guide grooves, which is the so-called track center. A laser beam for reproduction tracks the track center, whereby information recorded in the optical information recording disk can be reproduced.
FIG. 7 is a schematic partial perspective view of the above-mentioned convertional optical information recording disk, showing the geometrical relationship between the guide grooves, and prepits and recording pits which are formed along the track center thereof.
In FIG. 7, reference numeral 10 indicates the substrate of the optical information recording disk; reference numeral 11, a recording layer of the optical information recording disk; reference numeral 12, the guide groove which is preformed in the substrate 10; reference numeral 13, the prepit, which is also preformed in the substrate 10; reference numeral 14, the recording pit which is formed in the recording layer 11 provided on the substrate 10; and reference numeral 15, the track center along which the prepits 13 and the recording pits 14 are situated.
FIG. 3(a) shows the relationship between the prepits and the Rf signals thereof. More specifically, FIG. 3(a) indicates that the level of the Rf signal is shifted to a lower side below a slice level in each portion corresponding to the area in which the prepit is formed, while the level of the Rf signal is shifted to an upper side above the slice level in each portion corresponding to the area between the prepits where there are no prepits.
This is because the quantity of the light which is reflected by the recording layer and enters the photodetector is decreased by the diffraction by the prepits. The slice level for converting the Rf signal into a binary digital signal corresponds to a zero level (GND) when the Rf signal is subjected to AC coupling. This zero level is situated substantially at the center of the amplitude (peak to peak) of the Rf signal which is detected from the presence or absence of the prepits.
FIG. 5(a) shows the relationship between the Rf signal and the binary signal. The binary signal is high in the portion where the prepit is present, and low in the portion corresponding to the space between the prepits where there are no prepits.
FIG. 3(b) shows the relationship among the prepits, the recording pits (flag marks) and the Rf signal in the case where flag marks are normally recorded. Flag marks are recorded in the space between the prepits. The flag marks are also converted into binary signals in the same manner as in the case of the prepits by the reproduction system shown in FIG. 2. This slice level is also situated substantially at the center of the amplitude (peak to peak) of the Rf signals of the prepits. In the case of FIG. 3(b) in which the flag marks are normally recorded, since the pit level of the Rf signal of the flag mark is below the slice level, the flag mark can be normally detected as shown in FIG. 5(b).
The shortcomings of the conventional reproduction method, however, has the following shortcomings, which will now be explained with reference to FIG. 3(c) and FIG. 5(c).
FIG. 3(c) shows the case where the formation of recording pits is insufficient. It is considered that such insufficient formation of recording pits is caused by the decrease of a laser power during recording because of (1) the deterioration of a laser diode employed, (2) the recording under a defocused condition because of the deformation of an optical information recording disk employed, and/or (3) the presence of dust on the surface of the substrate of the optical information recording disk on the opposite side to the recording surface thereof (the recording is usually conducted by the application of a laser beam to the substrate side of the recording disk).
The size of a flag mark recorded under the above-mentioned abnormal conditions is smaller than the size of a normally recorded flag mark, so that the amplitude (B) of the Rf signal for the abnormally recorded flag mark is smaller than the amplitude (A) of the Rf signal for the normally recorded flag mark, that is, B&lt;A. In the case of FIG. 3(c), the pit level of the Rf signal for the flag mark does not reach the slice level, so that the flag mark does not appear as a binary signal as indicated in FIG. 5(c). As a result, even though the flag mark has been recorded, the flag mark cannot be detected.
Thus, in the conventional reproduction method, there is the risk that flag marks are erroneously detected, and if the flag marks are erroneously detected, the data registered by the user may be destroyed, and the data area cannot be handled during the reproduction thereof, so that correct reproduction of recorded data may accordingly become impossible.